title: Accidental introductions are an important source of invasive plants in the continental United States.
abstract: OBJECTIVE
  Preventing new plant invasions is critical for reducing large-scale ecological change. Most studies have focused on the deliberate introduction of nonnatives via the ornamental plant trade. However, accidental introduction may be an important source of nonnative, invasive plants.
  
  
  METHODS
  Using Web and literature searches, we compiled pathways of introduction to the United States for 1112 nonnative plants identified as invasive in the continental United States. We assessed how the proportion of accidentally and deliberately introduced invasive plants varies over time and space and by growth habit across the lower 48 states.
  
  
  RESULTS
  Deliberate introductions of ornamentals are the primary source of invasive plants in the United States, but accidental introductions through seed contaminants are an important secondary source. Invasive forbs and grasses are the most likely to have arrived accidentally through seed contaminants, while almost all nonnative, invasive trees were introduced deliberately. Nonnative plants invading eastern states primarily arrived deliberately as ornamentals, while a high proportion of invasive plants in western states arrived accidentally as seed contaminants. Accidental introductions may be increasing in importance through time. Before 1850, 10 of 89 (11%) of invasive plants arrived accidentally. After 1900, 20 of 65 (31%) arrived accidentally.
  
  
  CONCLUSIONS
  Recently enacted screening protocols and weed risk assessments aim to reduce the number of potentially invasive species arriving to the United States via deliberate introduction pathways. Increasing proportions of accidentally introduced invasive plants, particularly associated with contaminated seed imports across the western states, suggest that accidental introduction pathways also need to be considered in future regulatory decisions.
authors.affiliation: Department of Environmental Conservation, University of Massachusetts, Amherst, Massachusetts 01003, USA.
authors.forename: Nora E
authors.initials: NE
authors.lastname: Lehan
authors.forename: Julia R
authors.initials: JR
authors.lastname: Murphy
authors.forename: Lukas P
authors.initials: LP
authors.lastname: Thorburn
authors.forename: Bethany A
authors.initials: BA
authors.lastname: Bradley
mesh_terms: D003710:Demography; D004784:Environmental Monitoring; D058865:Introduced Species / Q000639:trends*; D057685:Plant Weeds / Q000145:classification / Q000502:physiology; D010944:Plants / Q000145:classification* / Q000235:genetics; D014481:United States
keywords: horticulture; introduction pathway; noxious weed; plant invasion; seed contaminant